1. Field of the Invention
The present invention relates to the field of geophysical survey data acquisition methods. More specifically, the invention relates to methods for acquiring data using randomized sources and receivers.
2. Description of the Related Art
This section of this document introduces various aspects of the art that may be related to various aspects of the present invention described and/or claimed below. It provides background information to facilitate a better understanding of the various aspects of the present invention. As the section's title implies, this is a discussion of “related” art. That such art is related in no way implies that it is also “prior” art. The related art may or may not be prior art. The discussion in this section of this document is to be read in this light, and not as admissions of prior art.
The performance of many geophysical surveys seeking valuable materials, such as hydrocarbon or minerals, often utilized signal sources and signal receivers. The sources generate certain type of energetic signal and receivers collect the signals which may be affected by the earth medium. The collected signals from the receivers are studied to determine the presence or absence of certain materials in the earth medium. Depending on the energy used for the surveys, there can be seismic surveys, Controlled Source Electromagnetic (CSEM) surveys, gravity surveys etc. Depending on the locations of the surveys, there can be surface seismic or borehole seismic surveys, where marine/offshore surveys, land/onshore surveys or transition zone surveys are several surface seismic survey examples, while Vertical Seismic Profile (VSP) survey is an example of borehole seismic surveys. Seismic survey is the most often used survey method in hydrocarbon exploration.
The performance of a marine seismic acquisition survey typically involves one or more vessels towing at least one seismic streamer through a body of water believed to overlie one or more hydrocarbon-bearing formations. WesternGeco L.L.C. currently conducts high-resolution Q-Marine™ surveys, in some instances covering many square kilometers. In many areas of the world hydrocarbon reservoirs located in structurally complex areas may not be adequately illuminated even with advanced towed marine streamer acquisition methods.
For example, the shallow, structurally complex St. Joseph reservoir off Malaysia produces oil and gas in an area that poses many surveying and imaging challenges. Strong currents, numerous obstructions and infrastructure, combined with difficult near-surface conditions, may hinder conventional survey attempts to image faults, reservoir sands, salt domes, and other geologic features.
A survey vessel known as a Q-Technology™ vessel may conduct seismic surveys towing multiple, 1000-10,0000-meter cables with a separation of 25-50 meters, using the WesternGeco proprietary calibrated Q-Marine™ source. “Q” is the WesternGeco proprietary suite of advanced seismic technologies for enhanced reservoir location, description, and management. For additional information on Q-Marine™, a fully calibrated, point-receiver marine seismic acquisition and processing system, as well as Q-Land™ and Q-Seabed™, see http://www.westerngeco.com/q-technology.
To achieve high density surveys in regions having a combination of imaging and logistical challenges, a high trace density and closely spaced streamers may be used. However, this presents the potential of entangling and damaging streamer cables and associated equipment, unless streamer steering devices are closely monitored and controlled. Wide-azimuth towed streamer survey data is typically acquired using multiple vessels, for example: one streamer vessel and two source vessels; two streamer vessels and two source vessels; or one streamer vessel and three source vessels. Many possible marine seismic spreads comprising streamers, streamer vessels, and source vessels may be envisioned for obtaining wide- or rich-azimuth survey data.
Assignee's co-pending application Ser. No. 11/335,365, filed Jan. 19, 2006, discusses some of these. This document discusses shooting and acquiring marine seismic data during turns of linear marine surveys and during curvilinear paths. While an advance in the art, the art continues to seek improvements to marine seismic data acquisition techniques.
The seismic wavefield W generated in seismic surveys is a function of seven independent variables:W=W(t,Xr,Yr,Zr,Xs,Ys,Zs)where:                t=time;        Xr=receiver sampling in the inline direction, or in the direction of the length of the streamer;        Yr=receiver sampling in crossline direction perpendicular to the inline direction;        Zr=receiver sampling in depth;        Xs=source sampling in the inline direction, or in the direction of the length of the streamer;        Ys=source sampling in crossline direction perpendicular to the inline direction; and        Zs=source sampling in depth.        
Conventional towed streamer acquisition in 2D, 3D, or wide-azimuth (“WAZ”) surveys is a parallel geometry, i.e., the receiver and the source lines are parallel. In towed streamer parallel marine acquisition the receiver are well sampled in the inline direction and not very well sampled in the crossline direction. For example, the inline receiver sampling might be 3.125 m to 12.5 m whereas the crossline receiver sampling might be 50 m to 200 m. The sources are also better sampled in the inline direction but poorly sampled in the crossline direction. So, inline source sampling might be 18.75 m to 150 m where crossline source sampling is 250 m to 600 m.
One characteristic of parallel geometry is that the data is regularly sampled because the sources and receivers are distributed in a regular grid. For marine towed streamer acquisition the planned (pre-plot) receiver locations can differ from the actual (post-plot) receiver locations despite efforts to control their position due to the effect of marine currents on the streamers. However, the source locations are always very regularly distributed.
This is important because, as one of the bedrock principles of seismic survey design, the sampling must meet what is known in the art as the “Nyquist criteria.” However, recent theoretical studies have shown that, if the seismic data is not sampled according to the Nyquist criteria (which is currently always the case in marine acquisition), it is better to have the data randomly sampled. Recently established principles of “compressive sampling” or “compressed sensing”, prove that reconstruction of images or signals can be done accurately with a smaller number of samples that required by Nyquist theory.
Recent research in theoretical application of compressive sampling to seismic data shows the potential benefits of random sampling for the reconstruction of seismic wavefields, Gilles Hennenfent & Felix J. Herrmann, “Random Sampling: New Insights Into the Reconstruction of Coarsely-Sampled Wavefields” SEG/San Antonio 2007 Annual Meeting, pp. 2575-2579 (2007), and for noise attenuation, Gilles Hennenfent & Felix J. Herrmann, “Simply Denoise: Wavefield Reconstruction Via Jittered Undersampling” 73 Geophysics V19-V28. (2008). However, nobody has yet achieved a mechanism by which such random sampling can be sufficiently achieved during an actual survey.
The art has also begun to develop an alternative to the conventional parallel geometry during acquisition. This was first suggested by Cole, R. A. et al., “A circular seismic acquisition technique for marine three dimensional surveys”, Offshore Technology Conference, OTC 4864, May 6-9, 1985, Houston, Tex., described a concentric circle shooting scheme for obtaining three dimensional marine survey data around a sub-sea salt dome. While perhaps useful when the location of the feature under survey is known, this technique would not be efficient or productive for finding new oil and gas deposits, or for monitoring changes in same if such information is desired.
A great leap in acquisition technology was described in another assignee's co-pending application Ser. No. 12/121,324, filed on May 15, 2008. This reference describes methods for efficiently acquiring wide-azimuth towed streamer seismic data, which is also known as the “coil shooting” technique.
For land seismic surveys, similar advancement in various aspects have been made with assignee's UniQ land system.
While the Q suite of advanced technologies for seismic data acquisition and processing may provide detailed images desired for many reservoir management decisions, including the ability to acquire wide- and/or rich azimuth data, the ability to acquire higher quality marine seismic data with less cost, or to increase the fold while also increasing the diversity of azimuth and offset, are constant goals of the marine seismic industry and would be viewed as advances in the art.